According to Car and Driver, Forza 3 offers more real-world tracks than GT5

[pariz]

Good for Forza then. I haven't played the game and I might never do it. I'm having a blast with GT5, which is by no means perfect, but its deep and huge and a very rewarding experience.

I just think it's good to know there's some competition out there that will demand developers to give more, try harder.